FIG. 1 depicts a schematic diagram of a telecommunications system in the prior art. Telecommunications system 100 comprises:                i. the Public Switched Telephone Network,        ii. private branch exchange 102,        iii. telecommunications terminals 103-1 and 103-2,        iv. telecommunications terminals 110-1 and 110-2, and        v. telecommunications terminal 111;all of which are interconnected as shown.        
The Public Switched Telephone Network (i.e., element 101) is a complex of telecommunications equipment that is owned and operated by different entities throughout the World. In the United States of America, for example, the Public Switched Telephone Network (or “PSTN”) comprises an address space that is defined by ten digits, and, therefore, comprises 10 billion unique addresses or “telephone numbers.” The Public Switched Telephone Networks in other countries are similar.
The Public Switched Telephone Network provides telecommunications service to telecommunications terminals 110-1, 110-2, and 111. Telecommunications terminals 110-1, 110-2, and 111 can be either wireline terminals or wireless terminals, or a combination of both.
Private branch exchange 102 is capable of switching incoming calls (e.g., from terminal 111, etc.) from the Public Switched Telephone Network via one or more transmission lines to terminals 103-1 and 103-2. Private branch exchange 102 is also capable of handling outgoing calls from terminals 103-1 and 103-2 to the Public Switched Telephone Network via one or more transmission lines.
Private branch exchange 102 is capable of also extending an incoming call (e.g., from terminal 111, etc.) to a telephone number of an “off-premises” terminal in the Public Switched Telephone Network, in addition to switching the same incoming call to an “on-premises terminal” within the enterprise area (e.g., an office building, etc.) that is served by exchange 102. Terminals 103-1 and 103-2 are considered to be on-premises terminals with respect to private branch exchange 102, while terminals 110-1 and 110-2 are considered to be off-premises terminals.
To accomplish the switching of an incoming, enterprise-related call to one of on-premises terminals 103-1 and 103-2, and the extending of the call to one of off-premises terminals 110-1 and 110-2, private branch exchange 102 maintains a table that correlates the off-premises telephone number to the on-premises, private branch exchange (PBX) extension. Table 1 depicts a table that illustrates the correlation.
TABLE 1PBX Extension-to-PSTN Number DatabaseOn-PremisesTelecommunicationsPrivate BranchTelecommunicationsTerminalExchange ExtensionNetwork Number103-1732-555-0102, x11201-555-1236103-2732-555-0102, x12908-555-3381. . .. . .. . .
As an example, a caller at terminal 111 who wishes to reach the PBX user of terminal 103-1 dials the PBX number (i.e., 732-555-0102). Private branch exchange 102 receives the incoming call, including the extension number (i.e., x11). Using information stored in memory and similar to what is shown in Table 1, private branch exchange 102 determines that the call is also to be extended to off-premises telephone number 201-555-1236, associated with terminal 110-1. The off-premises number corresponds to the off-premises terminal (e.g., a cellular phone, a home phone, etc.) that belongs to the PBX user of terminal 103-1. The idea behind transmitting the call to both terminal 103-1 and 110-1 is that if the PBX user is not reachable at his office phone (i.e., terminal 103-1), then maybe he is reachable at a phone that is outside of the office (i.e., terminal 110-1).
One problem that can occur, however, is that the extended call might ultimately be routed to a voice mail system (i.e., a non-enterprise system) that is not associated with private branch exchange 102. For example, if private branch exchange 102 attempts to reach cellular terminal 110-1, the cellular voice mail system of cellular terminal 110-1 might instead answer the call. Consequently, some calls that are made to the PBX user might go to the non-enterprise voice mail system. Meanwhile, other calls that are made to the PBX user might still go to the PBX voice mail system (i.e., the enterprise system), such as when the PBX user disables the call-extending feature when the feature is not needed. It is undesirable for enterprise-related calls to end up in the non-enterprise voice mail system because it burdens the PBX user with having to retrieve voice mail from multiple systems. Also, when accessing the non-enterprise voice mail system, the PBX user is inconvenienced with having to sort out the personal calls that were made directly to the user's off-premises terminal from the enterprise-related calls that were routed from the PBX.
What is needed is a technique to minimize the handing of enterprise-related calls by non-enterprise voice mail systems, without some of the disadvantages in the prior art.